The invention will be described with reference to providing protective and decorative finishes on exterior automotive panels, but it should be understood that the automobile is only one of many substrates to which the sheet material of the invention can be applied.
As pointed out in an article by Alan J. Backhouse entitled "Routes To Low Pollution Glamour Metallic Automotive Finishes", Journal of Coatings Technology, Vol. 54, No. 693, pages 83-90, October 1982, there is a growing need to reduce the amount of atmospheric pollution caused by solvents emitted during industrial painting processes. Many different approaches have been proposed. For example, efforts have been made to replace the solvent-based paints used for automobiles with water-based paints. Work has also been done on the use of high solids formulations to lessen the emission of organic solvents. However, the application of automotive finishes is a highly demanding art because of the extremely high quality of the surface finish required and because of the common application of metallic finishes to provide what Backhouse refers to as "high stylistic effects". Accordingly, past efforts to replace the low viscosity, low-solids-content paint formulations conventionally used in spray painting operations in the automotive industry have met with limited success.
A more promising approach is to eliminate entirely the need for spray painting. Elimination of spray painting, or reduction in its use, would not only reduce atmospheric pollution, but would provide cost savings in that spray painting operations are so wasteful that more than half of the paint may be wasted. A means for achieving such goal exists through the use of a pre-formed thermoplastic sheet material which can be bonded to the panel to provide the protective and decorative coating. Such techniques have been utilized for interior automobile panels as described, for example, in U.S. Pat. No. 3,551,232 issued Dec. 29, 1970.
The objective of U.S. Pat. No. 3,551,232 is to overcome the problems of bubbling and blistering of the resin sheet that tend to occur in the vacuum-forming process. It achieves this by use of an adhesive containing an inert particulate filler which minimizes the entrapment of air.
To employ a process of the type described in U.S. Pat. No. 3,511,232 with exterior automotive panels presents a greater challenge. The surface appearance of such panels is of critical importance, so that it is necessary not only to avoid bubbling or blistering caused by entrapped air, but to provide a protective and decorative coating that will equal or exceed the quality of a spray-painted surface. Furthermore, exterior automotive panels present a particular problem in view of the difficulty of smoothly adhering a flexible sheet material to a curved substrate and the difficulty of doing so while maintaining over the entire surface a uniform color intensity.
Efforts have been made by others to produce a paint-coated flexible and stretchable sheet material having these capabilities. In general, it is believed that these efforts have resulted in products having numerous coating defects and in which the paint layer does not have the high degree of uniformity which permits the sheet material to undergo the stresses of thermoforming and yet meet the exacting standards of an exterior automotive finish.
A new type of flexible and stretchable sheet material has been developed that is (1) capable of meeting the requirements of an exterior automotive finish and (2) capable of withstanding the stretching and bending forces involved in bonding it to exterior automotive panels. It is produced by laminar flow coating techniques which permit exacting control of the thickness and uniformity of the coatings. Such coating techniques provide essentially defect-free coatings having a substantially uniform quality and appearance.
The new type of sheet material comprises a flexible carrier film, a protective and decorative paint layer, also known as a basecoat, adhered to one surface of the carrier film, and a transparent topcoat or clearcoat over the basecoat. The carrier film has heat-softening and tensile elongation properties which adapt it to use in the thermoforming process and the paint layer and topcoat have compatible heat softening and tensile elongation properties. As a result, the sheet material can undergo substantial elongation without crazing or delamination of the layers.
The present invention provides a further improvement in the new type of sheet material, in particular, with regard to a reduction in the tendency toward elastic recovery by the stretched material.